"I announce my retirement, thanks to everyone who was involved and cheered me on!" the Hungarian, who won both individual medleys as well as backstroke gold and silver at the Rio Games in 2016, wrote on Facebook.
"For thirty years, water was my home, a sanctuary where I found solace and strength," she wrote.
"From the moment I first set foot in the pool as a child, I knew I had discovered something magical. The water's cool embrace felt like a homecoming, a place where gravity ceased to exist and each arm stroke brought me closer to my dreams."
Hosszu won 42 world championship medals between 2009 and 2019. Her haul included nine long-course and 17 short course world titles.
She won medals in the medleys, backstroke and butterfly. The number of events she swum at championships earned her the nickname "The Iron Lady", which she embraced.
"As I progressed in my swimming career, I went from a curious child to one of the most successful female swimmers ever," she wrote.
"Each race I won was not only a testament to my hard work and dedication, but also a reflection of the many training hours, sacrifices made."
"I'll never forget the thrill of the race."
Hosszu retires as the world record holder in the long-course 100-meter individual medley.
